The forum errors are a pain... If it doesn't respond correctly upon posting, I copy the message (for safety) and reload the page to make sure it didn't actually post. But anyway, I thought that you should be able to delete the top post and therefore the thread (maybe only if you do it straight away, or maybe not - this does ring a bell).
But if not, you ask a moderator/admin to delete one for you. Although it's neater if they merge the threads, then the posts, so we mods don't see the deleted thread kicking about afterwards.
(I've done that for you, I hope it's OK i kept your minute later post with a like on it.)